Currently, the majority of all vehicles driven today use front-end accessory drive alternators that contain Lundell style rotors, also known as “claw pole” rotors. The rotor provides the alternator's magnetic field and rotates within the machine. The rotor includes a field coil made up of a number of insulated copper wires wrapped around an electrically insulated bobbin. The bobbin surrounds a steel hub, and also insulates the field coil from the steel pole pieces which sandwich the field coil to form north and south poles. The magnetic field is generated when the field coil is energized and a current flows through the wires.
In such claw pole rotors, it is preferable to incorporate the steel core or hub into the pole pieces. Stated another way, each pole piece includes one half of the steel center hub, thereby forming a single face-to-face contact region. This design is preferred because by reducing the number of contact regions or surfaces, the magnetic field strength of the rotor increases, which is proportional to the amount of power the alternator can provide to the vehicle system. In these designs, the insulating bobbin needs to be relatively rigid and sturdy in order to support winding of the field coil directly onto the bobbin. Both the central cylinder and the opposing end caps of the bobbin need to provide sufficient support to confine the winding of the field coil. The bobbin is therefore typically formed of a relatively thick material.
Unfortunately, such thick bobbins reduce the dissipation of heat in the rotor and occupy space that could be better used for additional field coil or steel. In particular, the field strength of the rotor and the power of the alternator is increased by increasing the size of the coil or by applying more current therethrough. However, as current increases the power dissipation in the form of heat that goes up at a rate that is squared, given by the governing equation P=I2R, where P equals the power dissipation due to heat, I equals current and R equals the resistance of the coil.
Thus, some rotor and bobbin designs have been proposed where the bobbin is formed of a relatively thin material. Several exemplary bobbins are described in copending U.S. application Ser. Nos. 10/264,778, 10/057,059 and 10/057,061, commonly held by the Assignee of the present invention and the disclosures of which are hereby incorporated by reference in their entirety. With a thin bobbin, the steel hub is formed as a cylinder, on which two bobbin end caps are situated and taped down. The tape and folded inner flaps of the end caps form the central cylinder portion of the bobbin insulating the steel hub from the field coil. In this way, the steel hub may thus form structural support for winding the field coil directly onto the bobbin formed by the end caps and tape. Unfortunately, this design does not permit use of pole pieces having the hub integrally formed therein.
Accordingly, there exist a need to provide a rotor having a bobbin that is thin to promote heat transfer and increase the percentage of field coil winding, which also allows for both winding of the field coil directly on the bobbin, as well as the use of pole pieces having the steel hub integrally formed therein.